Conventionally, supercharging devices are employed for vehicle engines that are used in cars, trucks, wheel loaders, wheel cranes, etc., that run on wheels, so that a high power output with a compact size can be provided while obtaining the acceleration. As such supercharging devices (superchargers), there are a mechanical supercharging system, which employs a part of an engine's output or power from another mechanism, and a turbocharger system that employs exhaust gas. In the mechanical supercharging system, a compressor, such as a Roots blower supercharger, which is commonly employed, is mechanically driven by an engine via a belt, etc.
In a conventional mechanical supercharging system, however, meaningless supercharging at a predetermined level or higher is performed within a high-speed and low-load range, even though it is not necessary. The mechanical power loss due to the supercharger is thus increased, and the overall efficiency of the engine is degraded. In a supercharging system that uses a turbocharger, the acceleration for the vehicle is inferior when the engine acceleration is begun at a low idle rotational state. Since the exhaust energy provided by the engine is low and the rotation of the turbocharger is reduced accordingly, the volume of the pressurized air that is charged to the engine in this state is small, and a high torque can not be generated. As a result, turbo lag occurs, there is a slight delay before the vehicle begins to accelerate, and the operator feels that the vehicle is not responding well. In addition, when a high output engine that provides little exhaust is adapted for use with a supercharger, engine braking efficiency is reduced.